[linux-lvm] Does LVM have any plan/schedule to support btrfs in fsadm

[linux-lvm] Does LVM have any plan/schedule to support btrfs in fsadm

[heming.zhao]

Hello Zdenek & David,

 From URL: https://fedoraproject.org/wiki/Btrfs
The btrfs becomes default filesystem for desktops.

Do we have any plan to add btrfs code for scripts/fsadm.sh?

If the answer is yes. I could share a suse special patch, this patch
had been run about 4 years in suse products.

Thanks,
Heming

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Re: [linux-lvm] Does LVM have any plan/schedule to support btrfs in fsadm

[Zdenek Kabelac]

Dne 25. 06. 21 v 7:28 heming.zhao@suse.com napsal(a):
> Hello Zdenek & David,
>
> From URL: https://fedoraproject.org/wiki/Btrfs
> The btrfs becomes default filesystem for desktops.
>
> Do we have any plan to add btrfs code for scripts/fsadm.sh?
>
> If the answer is yes. I could share a suse special patch, this patch
> had been run about 4 years in suse products.


Hi

If you have some patches provided with some good usable testing (lvm2 test 
suite)  - it could be possibly merged.

On the other hand helping/suggesting users to use Btrfs on top of lvm2 has 
also its logical 'question' marks.  Since  btrfs users should probably be 
avoiding placing 'another' layer between real hw - when btrfs should be mostly 
capable handling lvm2 features in its 'very own' way. Each layer has it's own 
measurable costs. And yeah we do not want to be involved into btrfs related 
recovery cases, as we simply never understood its handling of attached disks.

Regards

Zdenek

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Re: [linux-lvm] Does LVM have any plan/schedule to support btrfs in fsadm

[heming.zhao]

On 6/25/21 6:57 PM, Zdenek Kabelac wrote:
> Dne 25. 06. 21 v 7:28 heming.zhao@suse.com napsal(a):
>> Hello Zdenek & David,
>>
>> From URL: https://fedoraproject.org/wiki/Btrfs
>> The btrfs becomes default filesystem for desktops.
>>
>> Do we have any plan to add btrfs code for scripts/fsadm.sh?
>>
>> If the answer is yes. I could share a suse special patch, this patch
>> had been run about 4 years in suse products.
> 
> 
> Hi
> 
> If you have some patches provided with some good usable testing (lvm2 test suite)  - it could be possibly merged.

Patch for fsadm is already many years ago, but test suite is empty.
I plan to add some test cases then file the patch files.

> 
> On the other hand helping/suggesting users to use Btrfs on top of lvm2 has also its logical 'question' marks.  Since  btrfs users should probably be avoiding placing 'another' layer between real hw - when btrfs should be mostly capable handling lvm2 features in its 'very own' way. Each layer has it's own measurable costs. And yeah we do not want to be involved into btrfs related recovery cases, as we simply never understood its handling of attached disks.
> 

In my opinion, the using style of btrfs by many users are same as ext4/xfs.
btrfs has lots of features, but there are MAY experimental or not stable.
Users prefer to use btrfs core functionality. And not to speak of compatible
reasons, in many scenarios, btrfs still works with lvm or mdadm.

For minimal code design (minimal bug occur), in fsadm, we only treat btrfs as
another ext4/xfs style filesystem.
"fsadm resize" executes "btrfs resize"
"fsadm check" executes "btrfs scrub"

Thanks,
Heming


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Re: [linux-lvm] Does LVM have any plan/schedule to support btrfs in fsadm

[Stuart D Gathman]

On Mon, 28 Jun 2021, heming.zhao@suse.com wrote:

> In my opinion, the using style of btrfs by many users are same as ext4/xfs.

Yes.  I like the checksums in metadata feature for enhanced integrity
checking.

It seems too complicated to have anytime soon - but when a filesystem
detects corruption, and is on an LVM (or md) RAID1 layer, an ioctl to
read alternate mirror branches to see which (if any) has the correct
data would allow recovery.  Btrfs does this if it is doing the
mirroring, but then you lose all the other features from LVM or md raid10, 
including running other filesystems and efficient virtual disks for
virtual machines.

We eventually got DISCARD operations to pass to lower layers.
Dealing with mirror branches should really be a thing too.

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Re: [linux-lvm] Does LVM have any plan/schedule to support btrfs in fsadm

[Gionatan Danti]

Il 2021-06-28 05:28 Stuart D Gathman ha scritto:
> Yes.  I like the checksums in metadata feature for enhanced integrity
> checking.

Until recently btrfs has issue when a LVM snapshot was mounted. It is 
now solved?

> It seems too complicated to have anytime soon - but when a filesystem
> detects corruption, and is on an LVM (or md) RAID1 layer, an ioctl to
> read alternate mirror branches to see which (if any) has the correct
> data would allow recovery.  Btrfs does this if it is doing the
> mirroring, but then you lose all the other features from LVM or md
> raid10, including running other filesystems and efficient virtual
> disks for
> virtual machines.

For this to work, LVM should be able to identify the corrupted data. 
Without checksum, how can you do that? The solution is to use 
dm-integrity under the RAID layer. It works pretty well, letting apart 
the big performance drop in the default (journaled) configuration 
(bitmap is faster, but leave a small window for corruption to happen 
undetected).

That said, for rewrite-heavy workload (virtual machines, databases, etc) 
btrfs is very slow (and disabling CoW is not a solution for me, as it 
also disables checksum, compression, etc).

> We eventually got DISCARD operations to pass to lower layers.
> Dealing with mirror branches should really be a thing too.

As said above, the issue is not to read from the mirror leg separately; 
rather, to detect *which* mirror leg contains valid data.

Regards.

-- 
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti@assyoma.it - info@assyoma.it
GPG public key ID: FF5F32A8

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Re: [linux-lvm] Does LVM have any plan/schedule to support btrfs in fsadm

[Chris Murphy]

On Mon, Jun 28, 2021 at 1:33 AM Gionatan Danti <g.danti@assyoma.it> wrote:
>
> Il 2021-06-28 05:28 Stuart D Gathman ha scritto:
> > Yes.  I like the checksums in metadata feature for enhanced integrity
> > checking.
>
> Until recently btrfs has issue when a LVM snapshot was mounted. It is
> now solved?

Pretty sure it's fixed since 4.14.
https://lkml.org/lkml/2019/2/10/23


> That said, for rewrite-heavy workload (virtual machines, databases, etc)
> btrfs is very slow (and disabling CoW is not a solution for me, as it
> also disables checksum, compression, etc).

It's not inherently slow, it's a tracking cost problem as very large
numbers of extents accumulate. And it also depends on the write
pattern of the guest file system. If you use Btrfs in a guest on a
host using Btrfs, it's a lot more competitive. There's certainly room
for improvement, possibly with some hinting to avoid writing out a
metric ton of 4KiB blocks as other file systems are prone to doing,
where btrfs can turn these into  largely sequential writes, they lose
any locality optimization the guest file system expects for subsequent
reads. A lot of the locality issue is a factor on rotational devices.
When talking about hundreds of thousands of extents per VM file, this
has a noticeable impact on even SSDs, but the much reduced latency
makes it tolerable for some scenarios.

But I've seen similar problems with VM's on LVM thinp when making many
snapshots and incurring cow, however temporary (like a btrfs nodatacow
file that's subject to snapshots or reflink copies; or a backing file
on xfs likewise reflink copied). There really isn't much better we can
do than LVM thick in this regard. And if that's the standard bearer,
it's not much different if you fallocate a nodatacow file.

Some databases are cow friendly, notably rocksdb. And sqlite with wal
enabled is at least not cow unfriendly. The worst offender seems to be
postgresql but I haven't seen any benchmarking since the multiple
kernel series of fsync work done on btrfs to improve the performance
of databases in general; that was kernel 5.8 through 5.11.


-- 
Chris Murphy

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Re: [linux-lvm] Does LVM have any plan/schedule to support btrfs in fsadm

[Gionatan Danti]

Il 2021-06-29 01:00 Chris Murphy ha scritto:
> Pretty sure it's fixed since 4.14.
> https://lkml.org/lkml/2019/2/10/23

Hi Chris, the headline states "harden against duplicate fsid". Does it 
means that the issue is "only" less likely or it was really solved?

> It's not inherently slow, it's a tracking cost problem as very large
> numbers of extents accumulate. And it also depends on the write
> pattern of the guest file system. If you use Btrfs in a guest on a
> host using Btrfs, it's a lot more competitive. There's certainly room
> for improvement, possibly with some hinting to avoid writing out a
> metric ton of 4KiB blocks as other file systems are prone to doing,
> where btrfs can turn these into  largely sequential writes, they lose
> any locality optimization the guest file system expects for subsequent
> reads. A lot of the locality issue is a factor on rotational devices.
> When talking about hundreds of thousands of extents per VM file, this
> has a noticeable impact on even SSDs, but the much reduced latency
> makes it tolerable for some scenarios.

I think the main issue stems for btrfs striking to have 4K CoW extents.
ZFS has a default 128K recordsize that, while commanding a fair 
read/modify/write overhead, works much better with HDDs (for SSDs one 
can lower recordize to 16K or 32K).
XFS with reflink does something similar, doing CoW at 128K block 
granularity (we had a similar discussion in the past: 
https://www.spinics.net/lists/linux-xfs/msg35679.html)

> But I've seen similar problems with VM's on LVM thinp when making many
> snapshots and incurring cow, however temporary (like a btrfs nodatacow
> file that's subject to snapshots or reflink copies; or a backing file
> on xfs likewise reflink copied). There really isn't much better we can
> do than LVM thick in this regard. And if that's the standard bearer,
> it's not much different if you fallocate a nodatacow file.

If I remember correctly thin LVM minimum chunk size should be 64K, 
making it much less prone to fragmentation. Moreover, it only CoW when a 
snapshot if overwritten for the first time (ZFS reallocates at each 
write and I think btrfs does something similar).

In a distant past, I benchmarked a virtual machine running on btrfs over 
a fallocated+nocow files and the result was quite bleak. Maybe things 
have improved more than I can imagine... time for some more benchmark I 
suppose! Do you have any to share?

> Some databases are cow friendly, notably rocksdb. And sqlite with wal
> enabled is at least not cow unfriendly. The worst offender seems to be
> postgresql but I haven't seen any benchmarking since the multiple
> kernel series of fsync work done on btrfs to improve the performance
> of databases in general; that was kernel 5.8 through 5.11.

Yeah, both PostgreSQL and MySQL tend to be slow on btrfs.

Regards.

-- 
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti@assyoma.it - info@assyoma.it
GPG public key ID: FF5F32A8

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